U.S. patent number 7,830,442 [Application Number 10/426,907] was granted by the patent office on 2010-11-09 for compact economical lidar system.
This patent grant is currently assigned to Arete Associates. Invention is credited to Gregory Fetzer, Asher Gelbart, Andrew Griffis, Brian Redman, David Sitter.
United States Patent |
7,830,442 |
Griffis , et al. |
November 9, 2010 |
Compact economical lidar system
Abstract
A lidar pulse is time resolved in ways that avoid costly,
fragile, bulky, high-voltage vacuum devices--and also costly,
awkward optical remappers or pushbroom layouts--to provide
preferably 3D volumetric imaging from a single pulse, or full-3D
volumetric movies. Delay lines or programmed circuits generate
time-resolution sweep signals, ideally digital. Preferably,
discrete 2D photodiode and transimpedance-amplifier arrays replace
a continuous 1D streak-tube cathode. For each pixel a
memory-element array forms range bins. An intermediate optical
buffer with low, well-controlled capacitance avoids corruption of
input signal by these memories.
Inventors: |
Griffis; Andrew (Tucson,
AZ), Fetzer; Gregory (Tucson, AZ), Redman; Brian
(Silver Spring, MD), Sitter; David (Torrance, CA),
Gelbart; Asher (Tucson, AZ) |
Assignee: |
Arete Associates (Northridge,
CA)
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Family
ID: |
32599731 |
Appl.
No.: |
10/426,907 |
Filed: |
April 29, 2003 |
Prior Publication Data
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Document
Identifier |
Publication Date |
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US 20040119838 A1 |
Jun 24, 2004 |
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Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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60377323 |
Apr 30, 2002 |
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Current U.S.
Class: |
348/340;
348/231.99 |
Current CPC
Class: |
G01S
7/4861 (20130101); G01S 17/89 (20130101); G01S
7/486 (20130101); G01S 7/4865 (20130101) |
Current International
Class: |
H04N
5/225 (20060101) |
Field of
Search: |
;348/31,231.99,335,340
;356/4.01,5.01,609 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Selby; Gevell
Attorney, Agent or Firm: Lippman; Peter I.
Parent Case Text
RELATED PATENT DOCUMENTS
This document claims priority of U.S. provisional patent
application Ser. No. 60/377,323 filed on Apr. 4, 2002.
Closely related documents are other, coowned U.S. utility-patent
documents and references--hereby wholly incorporated by reference
into this document. Those documents are in the names of: Kent
Bowker et al., U.S. provisional patent application Ser. No.
5,467,122, "UNDERWATER IMAGING IN REAL TIME, USING SUBSTANTIALLY
DIRECT DEPTH-TO-DISPLAY-HEIGHT LIDAR STREAK MAPPING" and earlier
documents cited therein; as well as Ser. No. 6,400,396 B1,
"DISPLACED-BEAM CONFOCAL-REFLECTION STREAK TUBE APPARATUS WITH
STRIP-SHAPED PHOTOCATHODE, FOR IMAGING VERY SMALL OBJECTS AND
OBJECTS THEREIN", and also PCT publication PCT/US95/15491 "IMAGING
LIDAR SYSTEM WITH STRIP-SHAPED PHOTOCATHODE AND CONFOCAL
REFLECTION"; and
Arete Associates, PCT publication PCT/US01/13489, entitled "MS-STIL
LIDAR".
Other patents and literature of interest, also wholly incorporated,
include: Frederick K. Knight et al., U.S. Patent Re. 33,865,
"DETECTOR FOR THREE-DIMENSIONAL OPTICAL IMAGING; Robert R. Alfano
et al., U.S. Pat. No. 5,142,372, "THREE-DIMENSIONAL OPTICAL IMAGING
OF SEMI-TRANSPARENT AND OPAQUE OBJECTS USING ULTRASHORT LIGHT
PULSES, A STREAK CAMERA AND A COHERENT FIBER BUNDLE"; D. V. Plant,
et al., "A 256 Channel Bi-Directional Optical Interconnect Using
VCSELs and Photodiodes on CMOS", Optics in Computing 2000, SPIE
Vol. 4089; E. M. Strzelecka, et al., "VCSEL Based Modules for
Optical Interconnects", SPIE Conference on Vertical-Cavity
Surface-Emitting Lasers III, SPIE Vol. 3627, Jan. 1999; J. Kim, et
al., "32.times.16 CMOS Smart Pixel Array for Optical
Interconnects", Optics in Computing 2000, SPIE Vol. 4089; J. Jiang
Liu, et al., "Multi-Channel Parallel Free-Space VCSEL
Optoelectronic Interconnects for Digital Data Transmission and
Processing", Proceedings of SPIE Vol. 4046 (2000); Jim Tatum and
Jim Guenter, "Modulating VCSELs", Honeywell Application Sheet,
February 1998; and Philip Hobbs, Building Electro-Optical Systems.
Claims
What is claimed is:
1. A lidar system comprising: means for generating a measurement
signal that is at least one-dimensional, corresponding to a
received at-least-one-dimensional lidar-beam pulse; means for
time-resolving the measurement signal, said resolving means
comprising: multiple memory elements for receiving and holding
successive portions of the measurement signal respectively, digital
means for forming a digital sweep signal defining multiple digital
states corresponding to the respective memory elements, and means
for applying the digital sweep signal to control distribution of
the successive measurement-signal portions into the respective
memory elements; wherein the forming means comprise a logic circuit
generating a series of digital pointers addressing the memory
elements respectively; means for reading the measurement-signal
portions from the memory elements; and multiple buffer switches
transferring the successive measurement-signal portions to the
multiple memory elements respectively; each buffer switch having a
respective enable terminal actuated by a respective one of the
digital pointers.
2. The system of claim 1, further comprising: multiple
electrooptical converters respectively receiving the successive
measurement-signal portions from the buffer switches, respectively,
and in response generating corresponding optical signals; and
multiple optoelectronic converters receiving the corresponding
optical signals and in response generating new corresponding
measurement-signal portions for application to the multiple memory
elements.
3. The system of claim 2, wherein: the electrooptical converters
are selected from the group consisting of VCSELs, LEDs, and organic
LEDs.
4. The system of claim 2, wherein: the optoelectronic converters
are selected from the group consisting of CMOS elements, organic
phase-shift molecular devices, and a printed-circuit stack of
thin-film devices.
5. A lidar system comprising: means for generating a measurement
signal that is at least one-dimensional, corresponding to a
received at-least-one-dimensional lidar-beam pulse; means for
time-resolving the measurement signal, said resolving means
comprising: multiple memory elements for receiving and holding
successive portions of the measurement signal respectively, digital
means for forming a digital sweep signal defining multiple digital
states corresponding to the respective memory elements, and means
for applying the digital sweep signal to control distribution of
the successive measurement-signal portions into the respective
memory elements; and means for reading the measurement-signal
portions from the memory elements; wherein: the forming means
comprise a tapped delay line having multiple taps addressing the
multiple memory elements respectively.
6. The system of claim 5, wherein: the memory elements comprise a
dynamic RAM or other capacitive array receiving the measurement
signal-portions substantially directly from the distribution
controlled by the delay-line taps.
7. The system of claim 5, further comprising: multiple buffer
switches transferring the successive measurement-signal portions to
the multiple memory elements respectively; each buffer switch
having a respective enable terminal actuated by a respective one of
the delay-line taps.
8. The system of claim 7, further comprising: multiple
electrooptical converters respectively receiving the successive
measurement-signal portions from the buffer switches, respectively,
and in response generating corresponding optical signals; and
multiple optoelectronic converters receiving the corresponding
optical signals and in response generating new corresponding
measurement-signal portions for application to the multiple memory
elements.
9. The system of claim 8, wherein: the electrooptical converters
are selected from the group consisting of VCSELs, LEDs, and organic
LEDs.
10. The system of claim 8, wherein: the optoelectronic converters
are selected from the group consisting of CMOS elements, organic
phase-shift molecular devices, and a printed-circuit stack of
thin-film devices.
11. A lidar system comprising: means for generating a measurement
signal that is at least one-dimensional, corresponding to a
received at-least-one-dimensional lidar-beam pulse; means for
time-resolving the measurement signal, said resolving means
comprising: multiple memory elements for receiving and holding
successive portions of the measurement signal respectively, digital
means for forming a digital sweep signal defining multiple digital
states corresponding to the respective memory elements, and means
for applying the digital sweep signal to control distribution of
the successive measurement-signal portions into the respective
memory elements; and means for reading the measurement-signal
portions from the memory elements; wherein: the forming means
comprise a delay line that comprises the memory elements; the delay
line itself has clock signals serving as the digital sweep signal;
and the delay line responds to the clock signals by successively
advancing the received successive measurement-signal portions into
the delay line.
12. The system of claim 11, wherein: the memory elements comprise a
dynamic RAM or other capacitive array receiving the measurement
signal-portions substantially directly from the distribution
controlled by the clock signals.
13. The system of claim 11, wherein: the delay line is a shift
register; the memory elements are successive positions in the shift
register itself; and the reading means comprise parallel circuits
for reading plural measurement-signal portions substantially
simultaneously from the shift register.
14. The system of claim 11, further comprising: an
analog-to-digital converter, digitizing the successive
measurement-signal portions for application to the delay line.
15. A lidar system comprising: means for generating a measurement
signal corresponding to a received lidar-beam pulse; means for
time-resolving the measurement signal; multiple electrooptical
converters respectively receiving time-resolved measurement-signal
portions from the resolving means, and in response forming new
corresponding optical signals; and means for reading the
measurement-signal portions as the new corresponding optical
signals from the electrooptical converters.
16. The system of claim 15, wherein: the electrooptical converters
are LEDs.
17. The system of claim 15, wherein: the electrooptical converters
are organic LEDs.
18. The system of claim 15, wherein: the electrooptical converters
are VCSELs.
19. The system of claim 18, further comprising: multiple
optoelectronic converters receiving the corresponding new optical
signals from the VCSELs and in response forming new corresponding
measurement-signal portions for readout by the reading means.
20. The system of claim 19, wherein: the optoelectronic converters
are CMOS elements.
21. The system of claim 19, wherein: the optoelectronic converters
are optical phase-shift molecules.
22. The system of claim 19, wherein: the optoelectronic converters
are printed-circuit stacks of thin-film devices.
23. The system of claim 15, further comprising: multiple
optoelectronic converters receiving the new corresponding optical
signals and in response forming new corresponding
measurement-signal portions for readout by the reading means.
24. The system of claim 23, wherein: the optoelectronic converters
are CMOS elements.
25. The system of claim 15, wherein: the resolving means comprise
multiple buffer switches directing the time-resolved
measurement-signal portions to the multiple electrooptical
converters, respectively; the multiple buffer switches comprise
respective enable terminals actuated by a synchronous enable
signal.
26. The system of claim 25, wherein: the synchronous enable signal
is substantially in controlled-delay synchronism with the
lidar-beam pulse.
27. The system of claim 25, wherein: before said synchronous enable
signal, each enable terminal is connected to receive a bias input
that holds the respective electrooptical converter just within a
quiescent state.
28. The system of claim 27, wherein: readout from the respective
electrooptical converter is terminated by another synchronous
signal after a time interval allowing for collection of the
time-resolved measurement-signal portion from that respective
electrooptical converter.
29. The system of claim 15, for detecting and ranging objects; said
system further comprising: means for projecting an
at-least-one-dimensional light pulse toward such objects; and means
for receiving an at-least-one-dimensional reflected light pulse
from such objects; wherein the generating means comprise means for
generating said measurement signal in response to the received
light pulse.
30. A lidar system comprising: means for generating an
at-least-one-dimensional measurement signal corresponding to an
at-least-one-dimensional received lidar-beam pulse; means for
time-resolving the measurement signal; multiple memory elements,
comprising a dynamic RAM or other capacitive array, respectively
receiving and holding time-resolved measurement-signal portions
substantially directly from the resolving means; and means for
reading the held measurement-signal portions from the memory
elements; and multiple buffer switches transferring the
time-resolved measurement-signal portions from the resolving means
substantially directly to the multiple memory elements
respectively; each buffer switch having a respective enable
terminal actuated by the resolving means.
31. The system of claim 30, for detecting and ranging objects; said
system further comprising: means for projecting an
at-least-one-dimensional light pulse toward such objects; and means
for receiving an at-least-one-dimensional reflected light pulse
from such objects; wherein the generating means comprise means for
generating said measurement signal in response to the received
light pulse.
Description
FIELD OF THE INVENTION
This invention relates generally to imaging; and more particularly
to a system and method using lidar (light detecting and ranging) to
characterize one or more objects or features in a medium.
BACKGROUND
Several techniques have evolved for dealing with the problems
associated with detecting objects in a light-scattering medium.
Many of these techniques and their drawbacks are discussed in the
above-referenced U.S. Pat. No. 5,467,122.
Other techniques include using an imaging system based on one or
more analog-to-digital converters (ADCs). In such a system, light
is first directed toward an object to be imaged.
Light that is reflected back from the object is then directed to a
comparator. If the comparator detects a requisitely large signal, a
state change is produced which in turn triggers an ADC to direct a
signal to a streak or charge-coupled device (CCD) camera for
imaging.
The drawbacks of this technique, however, are multifold. If a
single ADC is used in the system, it may entirely miss a
multiplicity of signals reflected back from juxtaposed objects. It
may, instead, form only a partial image of such objects based on
just the leading edge of a signal waveform.
Additionally, use of a single ADC restricts the imaging to very
small area coverage. Furthermore, even a single ADC is typically a
large bulky system and generates a great amount of heat. Thus, even
if several ADCs are used collectively to effectively increase the
range, both the size and heat accumulation issues remain to be
overcome. Dealing with these issues adds to the cost of producing
such systems, as does the fact that the applicable ADC systems are
not typically prefabricated. Most of the commercially available
ones are for physically small arrays or for very-large-scale
integration (VLSI).
Another problematic technique is commonly referred to as the "Magic
Lantern". For this technique, several intensified CCD cameras are
used to define range bins. Each camera is dedicated to a separate
piece of time referred to as a trigger image point.
This approach, however, leads to poor range resolution and area
coverage--especially for objects that are spaced far apart along
the range direction. As discussed at length in the previously
mentioned '122 patent, this is basically another range-gating
technique. Like the ADC system, it has the drawbacks of poor
ability to see clustered detail or to acquire multiple events over
a single lidar pulse.
Streak-tube lidar systems are generally based on the generation of
a periodic series of discrete pushbroom-shaped pulse beams to
illuminate an object in semiturbid medium. When reflected back, the
pulses are collected through a slit and onto a streak tube--which
is in turn coupled to an imaging detector such as a CCD for
imaging.
Such streak-tube lidar systems overcome many of the aforementioned
problems, but are subject to certain drawbacks. The streak tube
itself is a complex, bulky, expensive and relatively fragile
vacuum-tube device, requiring high voltages for both basic
operation and control.
As will be seen, these characteristics impose severe limitations
upon any effort to generate or use images in other formats or for
different purposes. The possible existence of such variations in
format and purpose are themselves considered part of the present
invention; hence these configurations will be introduced in a later
section of the present document.
What is needed, to realize such novel configurations and purposes,
is an imaging system that provides an accurate and reliable image
of an object in a light-scattering medium--and that not only
eliminates the problems associated with range-gating techniques and
bulky, heat-generating ADCs but is also relatively inexpensive to
produce as well as compact and more-easily transportable. Important
aspects of lidar imaging thus remain amenable to useful
refinement.
SUMMARY OF THE DISCLOSURE
The present invention introduces such refinement. The invention has
several main facets or aspects that can be used independently,
although for best enjoyment of their benefits certain of these
aspects or facets are preferably practiced in combinations
together.
In a first of these independent facets or aspects, the present
invention is a lidar system. The system includes some means for
generating a measurement signal that is at least one-dimensional,
corresponding to a received at-least-one-dimensional lidar-beam
pulse. For purposes of breadth and generality in discussion of the
invention, these means will be called simply the "generating
means".
This first main aspect of the invention also includes some means
for time-resolving the measurement signal. Again for generality and
breadth these means will be called the "resolving means". They
include: multiple memory elements for receiving and holding
successive portions of the measurement signal respectively, some
digital means for forming a digital sweep signal defining multiple
digital states corresponding to the respective memory elements (the
"forming means"), and some means for applying the digital sweep
signal to control distribution of the successive measurement-signal
portions into the respective memory elements (the "applying
means").
Also part of this first facet of the invention are some means for
reading the measurement-signal portions from the memory elements.
These means may be called the "reading means".
The foregoing may represent a description or definition of the
first aspect or facet of the invention in its broadest or most
general form. Even as couched in these broad terms, however, it can
be seen that this facet of the invention importantly advances the
art.
In particular, use of a digital sweep is one key to particularly
efficient, economical and cost-effective lidar systems that make
use of commercial, off-the-shelf fast modern electronics. Such
systems can replace the high-voltage, heavy, bulky and fragile
streak tube in all applications but the most extremely
demanding--in terms of range resolution.
Although the first major aspect of the invention thus significantly
advances the art, nevertheless to optimize enjoyment of its
benefits preferably the invention is practiced in conjunction with
certain additional features or characteristics. In particular,
preferably the forming means include a logic circuit generating a
series of digital pointers addressing the memory elements
respectively.
In event this basic preference is observed, then certain
subpreferences come into play. For example, first it is preferred
that the memory elements include a dynamic RAM or other capacitive
array receiving the measurement signal-portions substantially
directly from the distribution controlled by the digital
pointers.
A second such subpreference is that the system further include
multiple buffer switches transferring the successive
measurement-signal portions to the multiple memory elements
respectively. It is still further preferred that each buffer switch
have a respective enable terminal actuated by a respective one of
the digital pointers.
As an alternative preference to the basic logic-circuit preference
mentioned above (still for implementing the first main independent
aspect of the invention), the forming means preferably include a
tapped delay line having multiple taps addressing the multiple
memory elements respectively. If this second basic preference is
adopted, then certain subpreferences are applicable.
One of these is that the memory elements include a dynamic RAM or
other capacitive array receiving the measurement signal-portions
substantially directly from the distribution controlled by the
delay-line taps. Another subpreference is that the system further
include multiple buffer switches transferring the successive
measurement-signal portions to the multiple memory elements
respectively; and that each buffer switch have a respective enable
terminal actuated by a respective one of the delay-line taps.
In yet a third basic preference that can be used in place of the
first two just discussed, the forming means include a delay line
that includes the memory elements. Here the delay line itself has
clock signals serving as the digital sweep signal; and the delay
line responds to the clock signals by successively advancing the
received successive measurement-signal portions into the delay
line.
If the system is made to follow this third basic preference, then
once again corresponding subpreferences are of interest. One of
these is that the memory elements preferably include a dynamic RAM
or other capacitive array receiving the measurement signal-portions
substantially directly from the distribution controlled by the
clock signals.
Another subpreference is that the delay line be a shift register;
that the memory elements be successive positions in the shift
register itself; and that the reading means include parallel
circuits for reading plural measurement-signal portions
substantially simultaneously from the shift register. Yet another
subpreference is that the system preferably further include an
analog-to-digital converter, digitizing the successive
measurement-signal portions for application to the delay line.
Regardless of which of the three above-described basic preferences
(or other configurations) may be selected, certain additional
preferences are also applicable for use in the first main facet of
the invention. In particular preferably the system further
includes: multiple electrooptical converters respectively receiving
the successive measurement-signal portions from the buffer
switches, respectively, and in response generating corresponding
optical signals; and multiple optoelectronic converters receiving
the corresponding optical signals and in response generating new
corresponding measurement-signal portions for application to the
multiple memory elements.
As will be understood, by "electrooptical converters" is meant
devices that are driven by electrical input signals and produce
corresponding optical output signals; and by "optoelectronic
converters" is meant the converse--e. g., devices driven optically
to produce electronic output.
In this case, in turn preferably the electrooptical converters are
VCSELs, LEDs, or organic LEDs. At the same time, preferably the
optoelectronic converters are CMOS elements, organic phase-shift
molecular devices, or a printed-circuit stack of thin-film
devices.
Also regardless of which of the three basic preferences is adopted,
preferably the system is specifically equipped for detecting and
ranging objects, and accordingly further includes some means for
projecting an at-least-one-dimensional light pulse toward the
objects; and some means for receiving an at-least-one-dimensional
reflected light pulse from the objects. Thus the previously
mentioned generating means are able to generate the measurement
signal in response to the received light pulse.
Now turning to a second of the main independent facets or aspects
of the invention: in preferred embodiments of this second facet,
the invention is again a lidar system having means for generating a
measurement signal corresponding to a received lidar-beam pulse,
and means for time-resolving the measurement signal.
Preferred embodiments of this second aspect of the invention also
include multiple electrooptical converters that respectively
receive time-resolved measurement-signal portions from the
resolving means. In response these converters form new
corresponding optical signals. Such converters may be recognized as
preferable features of the first main facet of the invention,
discussed above.
This system also includes some means for reading the
measurement-signal portions as the new corresponding optical
signals from the electrooptical converters. The foregoing may
represent a description or definition of the second aspect or facet
of the invention in its broadest or most general form.
Even as couched in these broad terms, however, it can be seen that
this facet of the invention importantly advances the art. In
particular, as will be seen the use of such electrooptical
converters offers a present, short-term-available solution to the
problem of controlling measurement capacitances. A variety of such
converters on the market makes this solution particularly
appealing.
This market availability is especially important because certain of
these devices are enjoying extensive ongoing development for the
telecommunications industry. Components introduced in this way are
configured in multiple-unit arrays with low cost and power but high
linearity--and consistent, controllable input and output
capacitance. These several properties make the devices ideal for
use in facilitating lidar-signal time resolution.
Although the second major aspect of the invention thus
significantly advances the art, nevertheless to optimize enjoyment
of its benefits preferably the invention is practiced in
conjunction with certain additional features or characteristics.
Many of these have been mentioned previously as preferences for the
first main facet of the invention.
In particular, preferably the electrooptical converters are
vertical cavity surface-emitting lasers ("VCSELs"). These are
devices only recently introduced as telecommunications components
but now readily available with all the favorable properties
mentioned in the preceding paragraph.
An alternative preference for the electrooptical converters, not
far behind in terms of the listed properties, are LEDs. Still
another preference is organic LEDs.
A second basic preference is that the system further include
multiple optoelectronic converters receiving the corresponding new
optical signals from the electrooptical converters (e. g. VCSELs)
and in response forming new corresponding measurement-signal
portions for readout by the reading means. If such optoelectronic
converters are used, one particularly preferable choice of such
devices CMOS elements. Another is optical phase-shift molecules,
and yet another is printed-circuit stacks of thin-film devices.
A further basic preference is that the resolving means include
multiple buffer switches directing the time-resolved
measurement-signal portions to the multiple electrooptical
converters, respectively. The multiple buffer switches
advantageously include respective enable terminals actuated by
synchronous enable signal.
Such an enable signal is "synchronous" in the sense of being
substantially in controlled-delay synchronism with the lidar-beam
pulse. Preferably, before arrival of the synchronous enable signal
each enable terminal is connected to receive a bias input that
holds the respective electrooptical converter just within a
quiescent state.
Also preferably readout from the respective electrooptical
converter is terminated by another synchronous signal. This signal
is provided after a time interval allowing for collection of the
time-resolved measurement-signal portion from that respective
electrooptical converter.
Regardless of the here-mentioned preferences selected for use with
this second aspect of the invention, it is preferable that the
system further include some means for projecting a light pulse
toward the objects; and some means for receiving a reflected light
pulse from the objects. Here it is also preferable that the
generating means include means is for generating the measurement
signal in response to the received light pulse.
In preferred embodiments of its third major independent facet or
aspect, the invention is a lidar system. It includes some means for
generating an at-least-one-dimensional measurement signal
corresponding to an at-least-one-dimensional received lidar-beam
pulse.
This system also includes some means for time-resolving the
measurement signal. In addition it includes multiple memory
elements, include a dynamic RAM ("DRAM") or other capacitive
array.
These memory elements respectively receive and hold time-resolved
measurement-signal portions substantially directly from the
resolving means. The system also includes some means for reading
the held measurement-signal portions from the memory elements.
The foregoing may represent a description or definition of the
third aspect or facet of the invention in its broadest or most
general form. Even as couched in these broad terms, however, it can
be seen that this facet of the invention importantly advances the
art.
In particular, such direct connection from the resolving means of
this aspect of the invention to the memory elements will eliminate
need for an intermediate isolating stage (such as passage through
the optical domain as described above for the second main facet of
the invention). This will make the overall system extremely
efficient, compact and low in power consumption.
Multiple memory-element devices of the DRAM or other
capacitive-array type are expected to become commercially available
very soon, under the impetus of the emerging fingerprint-analysis
industry. It is anticipated that such arrays will have very high
numbers of units per package, consistent and well-controlled
capacitance, high speed, low cost and other characteristics that
will enhance direct connection as described above.
Although the third major aspect of the invention thus significantly
advances the art, nevertheless to optimize enjoyment of its
benefits preferably the invention is practiced in conjunction with
certain additional features or characteristics. Preferably the
system further includes multiple buffer switches transferring the
time-resolved measurement-signal portions from the resolving means
substantially directly to the multiple memory elements
respectively. Each of the buffer switches has a respective enable
terminal actuated by the resolving means.
As in the first facet of the invention the system preferably
further includes some means for projecting an
at-least-one-dimensional light pulse toward the objects; and some
means for receiving an at-least-one-dimensional reflected light
pulse from the objects. The generating means include means for
generating the measurement signal in response to the received light
pulse.
In preferred embodiments of its fourth major independent facet or
aspect, the invention is a lidar system. The system includes some
means for generating a measurement signal corresponding to a
received lidar-beam pulse.
It also includes a delay line that accepts successive portions of
the measurement signal. The system also includes some means, within
the delay line, for advancing successively accepted signal portions
farther into the delay line.
Also included are some means for reading multiple
measurement-signal portions substantially simultaneously from
multiple positions along the delay line. The foregoing may
represent a description or definition of the fourth aspect or facet
of the invention in its broadest or most general form. Even as
couched in these broad terms, however, it can be seen that this
facet of the invention importantly advances the art.
In particular by applying a measurement signal directly to a delay
line, and then reading measurement-signal portions out of the delay
line for storage or use, the system tends to reduce the need for
extremely rapid readout to following stages. Instead the readout
from all the elements of the delay line can be performed relatively
more slowly--for example, after the entire reflected pulse has been
collected.
Although the fourth major aspect of the invention thus
significantly advances the art, nevertheless to optimize enjoyment
of its benefits preferably the invention is practiced in
conjunction with certain additional features or characteristics. In
particular, preferably the system further includes multiple memory
elements receiving the portions of the measurement signal
substantially directly from the multiple positions along the delay
line.
Such multiple memory elements, if present, most preferably take the
form of a dynamic RAM or other capacitive array. Another preference
is that the delay line take the form of a shift register, and that
the memory elements receive the successive measurement-signal
portions from successive stages of the shift register. In this case
the reading means include parallel circuits for reading plural
measurement-signal portions substantially simultaneously from the
shift register to the memory elements.
Another basic preference is that the system also include an
analog-to-digital converter ("ADC"), digitizing the successive
measurement-signal portions for application to the shift register.
In this case preferably both the ADC and the shift register are
plural-bit devices, enabling the fundamental measurements to be
made and recorded with substantial bit depth.
Also preferably the shift register is a CMOS device. A so-called
"sample & hold" delay line is particularly preferable to
minimize the number of separate components and stages.
Another preference is that the system still further include
multiple buffer switches transferring the measurement-signal
portions from the delay line substantially directly to the multiple
memory elements respectively. In this case each buffer switch has a
respective enable terminal that is actuated by a read signal--after
generation of the measurement signal is substantially complete.
The system advantageously is for detecting and ranging objects, and
accordingly further includes some means for projecting a light
pulse toward the objects; and some means for receiving a reflected
light pulse from the objects The generating means, in this event,
include means for generating the measurement signal in response to
the received light pulse.
In preferred embodiments of its fifth major independent facet or
aspect, the invention is a method for making three-dimensional
images of a volume, and features in the volume. This method uses a
two-dimensional array of multiple discrete photosensitive detectors
and electronic circuitry connected with said detectors.
The method includes the step of directing a two-dimensional lidar
pulse, reflected from the volume and features, to the array of
multiple discrete photosensitive detectors. Another step is
generation of a corresponding two-dimensional array of multiple
discrete electronic signals by the array of detectors.
Also included is the step of passing the entire resulting array of
signals from the photosensitive detectors to the electronic
circuitry. A further step is operating the electronic circuitry to
time-resolve the entire array of signals, thereby generating a
three-dimensional electronic image of the features.
The foregoing may represent a description or definition of the
fifth aspect or facet of the invention in its broadest or most
general form. Even as couched in these broad terms, however, it can
be seen that this facet of the invention importantly advances the
art.
In particular, this method can (but does not necessarily) provide
an entire three-dimensional electronic image of a complete volume,
based on just one single lidar pulse. Moreover, because the entire
electronics package can be solid-state and largely digital devices,
for the first time such equipment can be made light, compact, and
relatively economical--and also low in power consumption and heat
generation.
Although the fifth major aspect of the invention thus significantly
advances the art, nevertheless to optimize enjoyment of its
benefits preferably the invention is practiced in conjunction with
certain additional features or is characteristics. In particular,
preferably the operating step does in fact include generating the
entire three-dimensional electronic image of the features from
substantially a single lidar pulse.
Another preference is including the step of--before the directing
step--projecting a two-dimensional lidar pulse toward the volume
and features, to create the reflected two-dimensional lidar pulse.
Still another step is to include the step of--after the operating
step--using the three-dimensional image as a three-dimensional
representation of the features in the volume. This using step is
not necessarily immediate: it can instead include first storing the
three-dimensional electronic image; and later recovering the stored
image for later use as said three-dimensional representation of the
features.
In preferred embodiments of its sixth major independent facet or
aspect, the invention is a system for forming a three-dimensional
image of a volume, and features in the volume. The system includes
a two-dimensional array of multiple discrete photodetectors.
This detector array receives a two-dimensional lidar pulse
reflected from the volume and the features. In response the
detector array generates a two-dimensional array of corresponding
discrete electronic signals.
The system also includes a two-dimensional array of multiple
discrete electronic circuits connected to receive the array of
signals from the detector array. The circuits include some means
for time-resolving the entire array of signals, to generate from
the pulse a three-dimensional electronic image of the features.
The foregoing may represent a description or definition of the
sixth aspect or facet of the invention in its broadest or most
general form. Even as couched in these broad terms, however, it can
be seen that this facet of the invention importantly advances the
art.
In particular, this apparatus form of the invention is closely
related to the fifth, method, facet of the invention and shares the
same fundamental advances.
Although the sixth major aspect of the invention thus significantly
advances the art, nevertheless to optimize enjoyment of its
benefits preferably the invention is practiced in conjunction with
certain additional features or characteristics. In particular,
preferably the system further includes an optical source, which
projects a two-dimensional lidar pulse toward the volume and the
features, to create the reflected two-dimensional lidar pulse.
A group of preferences relates to choice of the type of
photodetector used. One such preference is that they include
avalanche photodiodes (APDs).
Another is that the detectors include positive intrinsic negative
(PIN) photodiodes. Yet another is that they include indium gallium
arsenide detectors.
Still another is that they include a charge-coupled device (CCD)
array. These choices are not necessarily mutually exclusive, as the
detector array can include more than one type--for example in
different regions of the detection field.
Another group of preference relates to the choice of electronic
circuits. These may include a two-dimensional array of
transimpedance amplifiers (TIAs) connected to receive the signal
array from the detectors and to drive the time-resolving means.
A different possibility is that the electronic circuits include a
two-dimensional array of operational amplifiers (op-amps)
configured for low-noise transimpedance signal gain. These op-amps
are connected to receive the signal array from the detectors and
drive the time-resolving means.
Still another option is that the circuits include a two-dimensional
array of transmission lines connected to receive the signal array
from the detectors, respectively; and a two-dimensional array of
microwave amplifiers fed by the transmission lines, respectively.
The transmission lines are connected to drive the time-resolving
means.
A further group of preferences addresses the makeup of the
time-resolving means. These may include--for handling successive
segments of the electronic signal from each detector--a respective
array of buffer amplifiers; together with a respective array of
time-controlled switches connected to actuate the buffer
amplifiers.
An additional preference in this case is that the system also
include a respective array of programmable logic circuits
generating time-base control signals to operate the switches.
Alternatively the system preferably includes a respective array of
delay lines generating time-base control signals to operate the
switches.
Another preference is that the time-resolving means incude--again
for handling the electronic signal from each detector--a respective
array of vertical-cavity surface-emitting lasers (VCSELs). The
VCSELs sample successive segments of the electronic signal from
each detector. A respective array of range-bin memory elements is
connected to receive and integrate signal samples from the
VCSELs.
Still another preference is that the system also include a handheld
portable case. The case encloses and carries substantially the
entire photodetector array and the electronic circuits.
As mentioned earlier, certain of the four main independent facets
or aspects of the invention are advantageously employed in
combination together, to maximize enjoyment of their respective
benefits. All of the foregoing operational principles and
advantages of the present invention will be more fully appreciated
upon consideration of the following detailed description, with
reference to the appended drawings, of which:
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is an elevational view of a single-pixel receiver element
according to a preferred embodiment of the invention that employs
VCSEL converters following a time-resolution stage;
FIG. 2 is a circuit diagram of a photodiode and TIA for wideband
performance with low-noise gain, according to a preferred
embodiment of the invention--which may be but is not necessarily
the FIG. 1 embodiment;
FIG. 3 is a graph of calculations for equivalent input
noise-current density as a function of input bandwidth, in the FIG.
2 embodiment;
FIG. 4 is a basic block diagram of a preferred embodiment for
optimizing detection;
FIG. 5 is a timing diagram for the FIG. 1 embodiment, particularly
providing an example of VCSEL and photodetector timing control;
is FIG. 6 is a circuit diagram of a preferred VCSEL drive (e. g.
for the FIG. 5 embodiment), wherein a separate bias and signal
output are incorporated for holding the VCSEL just below threshold
before ranging;
FIG. 7 is a photographic plan of: airborne (helicopter) flash-lidar
mosaic imagery of contrast (a) and range (b) for a van parked under
trees, further including a conventional photographic image of the
scene (c), representing flight lines for the image sequences shown
with direction arrows;
FIG. 8 is an elevational view, partially schematic, of a prior-art
STIL transceiver system engaged in a pushbroom sweep over a
target--also realizable with preferred embodiments of the present
invention;
FIG. 9 is a system block diagram for three-dimensional (3D) imaging
lidar according to a preferred embodiment of the invention;
FIG. 10 is an elevational view of a single-pixel receiver element
according to a preferred embodiment of the invention;
FIG. 11 is a photograph of a 4.times.4 VCSEL prototype on a
substrate (Honeywell, 2000);
FIG. 12 is a group of photographs showing (a) a TIA-VCSEL array
driven with flexible interconnect, (b) an example of a TIA-VCSEL
multidie cube, (c) a close-up of the TIA-VCSEL cube edge interface
and (d) a close-up of a VCSEL array, (Irvine Sensor Corporation,
2002);
FIG. 13 is an elevational view of a five-pixel (linear array in the
focal plane) receiver with N range pixels (bins) according to a
preferred embodiment of the invention (for simplicity the control
and digitizer electronics are omitted; however, for a person
skilled in this field these components are implicit in the
diagram);
FIG. 14 is an elevational view of a flash-lidar configuration
according to a preferred embodiment of the invention;
FIG. 15 is an elevational view of prior-art streak-tube technology
for generating 2D spatial-temporal images;
FIG. 16 is an elevational view of prior-art multislit streak-tube
imaging lidar--particularly used in a flash-lidar
configuration;
FIG. 17 is an elevational view of optimized components of a
preferred embodiment of the invention, (with closer views shown in
b through d);
FIG. 18 is a photograph of prior-art airborne STIL data: a
geodetically registered 3D map of power lines over a local
depression with canopy, and a grove of fruit trees visible at lower
right (2000, Oxnard, Calif.; 9 ns NdYag laser)--these types of
images, too (as well as those in FIGS. 19 through 22) being
obtainable with preferred embodiments of the present invention;
FIG. 19 is a like photo of a prior-art airborne 3D image of a
15.times.17 m school of fish in 35 m of water;
FIG. 20 is a like image of an underwater-vehicle contrast map of a
bottom scene with test objects; a 3D rendering of a manta-mine-like
object in the lower-right portion of that image is rendered in the
separate image to the right;
FIG. 21 is a group of like images of a prior-art STIL system
autonomous underwater vehicle (AUV) for underwater towed-vehicle
imaging;
FIG. 22 is a like image of a prior-art STIL technology demo for
industrial measurement; sub-millimeter resolution 3D image of
ping-pong ball over a support platform; 3D wireframe data generated
with STIL sensors were overlaid on a 3D model of the ball;
FIG. 23 is an elevational view of a prior art STIL system on a
moving platform as it views underwater objects; and
FIG. 24 is a combination of an elevational view and a block diagram
of prior-art STIL technology.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
Details of one preferred embodiment according to the invention
include:
1. a detection photodiode 112 (FIG. 1),
2. a transimpedance amplifier (TIA) 116,
3. a tapped delay line 24,
4. a vertical-cavity surface-emitting laser (VCSEL) 30, and
5. an image photodiode 12.
Detection Photodiode
This element 112 generates a measurement signal 11. The device can
be, merely by way of example, an avalanche photodiode (APD) or
positive-intrinsic-negative photodiode (PIN).
A difference between the two is that the APD can have significant
gain, acting as a photomultiplier, and the PIN cannot. Gains
exceeding one hundred are common for the APD.
A disadvantage of the APD, however, is that its gain comes at the
expense of good noise performance--a typical noise factor being
five (i.e., five times worse than shot-noise-limited performance).
Furthermore, the APD requires relatively high voltages for
operation.
As a result of its gain capability (and the associated
semiconductor structures), the APD is not readily combined with
other semiconductor devices on a single die. These several negative
factors may strongly motivate alternative designs that can use a
PIN photodiode instead of an APD.
The PIN photodiode does not mandate high voltages, but values on
the order of 10 V are relatively common--to help reduce diode
capacitance, for bandwidth optimization. The PIN is more easily
integrated with other technologies, and available in array
format.
APDs and PINs have similar quantum efficiencies, for given
semiconductor material. Hence a real issue to face in designing a
suitable PIN-based detector 12 is that of obtaining gains
equivalent to the APD, but equal to or better noise performance.
(Equivalent noise performance is still significant when integration
and scaling are considered.)
The accepted (and now best) way to obtain gain with a PIN is to use
the same gain stage that would be used for an APD. Such a stage
includes an op-amp 16 configured for transimpedance gain, or a
transimpedance amplifier (TIA) 116.
Transimpedance Amplifier (TIA)
In preferred embodiments, a TIA 116 (FIG. 2) is used to feed a
digitally swept time-resolving subsystem. The TIA is implemented
using an op-amp 16. (The noninverting case can be substituted, with
a slight loss of bandwidth.)
The TIA 116 is an op-amp 16 configured to amplify a current signal
while using it to develop a corresponding voltage; hence the name.
The TIA 116 can be customized to meet specific needs or uses of the
system. Principal areas of customization for the TIA 116 are: a.
minimizing the impact of the photodiode capacitance through
selection of applied bias voltage, or through isolation of the
corresponding op-amp 16 input using an active element such as a
common-base field-effect transistor (FET); b. optimizing gain and
overall bandwidth through the selection of op-amp 16 feedback
resistance and capacitance, taking into consideration the input
capacitances of both the photodiode and op-amp, and also the output
impedance expected looking into the load 38; and C. optimizing
noise performance by trading gain and bandwidth for noise figures,
taking into account the voltage and/or current noise referred to
the op-amp 16 input (these are often specified for an op-amp 16--at
least one or the other of voltage or current equivalent noise, in
per root-hertz units).
As to point "a" above, the goal--in terms of net input capacitance
presented to the op-amp 16 input--is to maximize the bandwidth
available for the TIA 116.
Depending on the specific application, greater emphasis may be
placed on minimizing noise or maximizing gain; hence some
application-specific tuning of the system design ("customization")
will occur. Here an op-amp 16 circuit (FIG. 2) from the Texas
Instruments (TI) OPA686 datasheet was used (see www.ti.com for more
information on this unit).
A primary concern is typical noise floor and bandwidth performance.
For exemplary discussion the OPA686 is helpful as it has served
well in a prototype of a preferred embodiment. This component has
low noise (approx. 1 nV/ Hz) and high gain bandwidth (1600
MHz).
Added noise for the inverting TIA 116 is a function of the
noninverting input-current noise I.sub.n, the input-voltage noise
V.sub.n, the feedback resistor R.sub.f, and the diode capacitance
C.sub.d, which for simplicity also includes the input capacitance
of the op-amp 16. The compensation capacitor is chosen for
bandwidth and stability, but does not directly affect noise
performance. Input-referenced added-noise current for this TIA 116
is: I.sub.eq= {square root over
(I.sub.n.sup.2+4kT/R.sub.f+(V.sub.n/R.sub.f).sup.2+0.33(2.pi.V.sub.nC.sub-
.df).sup.2)}{square root over
(I.sub.n.sup.2+4kT/R.sub.f+(V.sub.n/R.sub.f).sup.2+0.33(2.pi.V.sub.nC.sub-
.df).sup.2)}, where f is the output bandwidth of the amplifier 16.
This current is a function (FIG. 3) of output bandwidth for the
settings C.sub.f=0.8 pF, R.sub.f=10 kW and C.sub.d=50 pF. For
example this yields an equivalent input-noise current of 9 pA/ Hz
at 37 MHz output bandwidth--which at the same 37 MHz input
bandwidth results in a noise current of 55 nA.
Cast in terms of the input-referred current noise, the minimum
detectable signal 11 is then some multiple of the equivalent
current noise. This is easier to conceptualize if cast in terms of
power. To put this into context, assume that, as for streak-camera
170 applications, a minimum detectable return is on the order of
one hundred photoelectrons ("100 e.sup.-"). For a transmitted pulse
82 of 10 ns (10.sup.-8 sec) width, this would imply a current of:
I.sub.pulse=100e.sup.-1.610.sup.-19 C/e.sup.-/10.sup.-8
sec=1.610.sup.-9 A=1.6 nA.
Using the above amplifier configuration, these thoughts suggest
that the noise for the 37 MHz TIA 116 is more than thirty times
higher than a 100 e.sup.- signal for an equivalent streak-camera
response. From this it is clear that, even working with a
high-performance op-amp 16, obtaining wide bandwidth and low-noise
gain is a challenge.
Fortunately, this is a problem that has been addressed
significantly--but for wholly different applications and
marketplace--by existing photonics companies. For instance Analog
Modules (see www.analogmodules.com) has a commercially available
photodiode-TIA module that has been optimized for packaging and
component selection to provide wide bandwidth and low-noise
transimpedance gain (FIG. 17). This part, the P/N 713A-7, delivers
up to 200 MHz bandwidth at 18 V/mW input voltage and 8 pW/ Hz,
using an InGaAs PIN photodiode with 12 V applied bias.
For ease of comparison with other results discussed here, it may be
helpful to express the noise power in terms of noise current. The
power specifications can be converted to current with a few
assumptions. If it is assumed (as many Analog Modules application
notes assume) that a 50 .OMEGA. output impedance is nominal, then 8
pW at the output can produce 400 nA into 50 ohms, or 20 .mu.V
across that same output load 38.
Using the inferred 20 k.OMEGA. feedback gain (from datasheet
notes), this implies input current of 1 nA. The 8 pW/ Hz thus
becomes 1 nA/ Hz equivalent input-current noise. For these
assumptions, such a low noise figure represents a significant
improvement over the stock-configuration TI transimpedance design
(FIG. 2).
A front-end receiver/system based on the photodiode-TIA module made
by Analog Modules has been demonstrated and is further described
herein (FIG. 3). Several preferred embodiments encompass
alternative front-end stages that will be straightforward, for
workers skilled in this field, in view of the introduction in this
document.
An example of one such approach is to direct the output of the
diode to a transmission line that feeds a microwave amplifier 14.
The noise-limited performance in this case is just a matter of
determining the desired bandwidth and the noise figure of the
amplifier. Minimum detectable signal for a microwave amplifier 14
is given by-- M.sub.EDS=kTBF, where F is the amplifier noise
figure, T the physical temperature, k Boltzman's constant and B the
bandwidth in hertz. For instance, at 300 K and 100 MHz bandwidth,
and using an amplifier 16 with a noise figure of 3 dB (10 log F),
the MDA is: M.sub.EDS=8.2810.sup.-13W=-91 dBm (dBm is dB with
respect to 1 mW). This translates to a minimum detectable current
and voltage (at 50.OMEGA. input impedance) of-- I.sub.min= {square
root over (M.sub.EDS/Z)}= {square root over (8.2810.sup.-13
/50)}=1.310.sup.-7 A=130 nA V.sub.min= {square root over
(ZM.sub.EDS)}=3.910.sup.-6 V, or 3.9 .mu.V.
This analysis explains why it is not common practice to connect a
low-noise microwave amplifier 14 to a diode and hope that it will
work. It also suggests, however, that there is a trade space in
which to work, provided some customization is introduced--and the
present inventors have confirmed this suggestion.
Specifically, as the noise figure depends on the impedance match
between current source and amplifier 16 at 50 .OMEGA., with
associated assumptions about the real part of that impedance, there
is generally room to make tradeoffs. This amounts to optimizing the
amplifier input impedance (Z.sub.amp) 41 (FIG. 4) to yield the
lowest noise figure when the unit is connected to a photodiode
characteristic impedance (Z.sub.pd) 40.
This approach, used earlier for telecommunication applications, has
been validated for use with the present invention. Nevertheless the
circumstances are different enough (essentially coherent detection)
that many of the assumptions are less applicable to the invention;
hence the theory outlined bears reanalysis--even notwithstanding
success in actual operation by the present inventors.
Tapped Delay Line
As noted earlier, the system requires some means for time-resolving
the digital signal. In preferred embodiments of the invention a
digital sweep is applied to control the distribution of successive
signals 11.
Initially it was thought that a traditional analog tapped delay
line 24 could be used to directly feed a VCSEL 30 array--with some
help from so-called buffer "enables" (i.e., forward-transmission
enabling terminals) and the like is in the VCSEL drive circuit.
Such configurations remain of interest for specific applications,
as will occur to artisans skilled in this field.
Working with typical analog tapped delay lines 24, however, is
relatively clumsy--as those devices tend to be tuned for very
specific delays, and use passive/reactive circuit elements to
provide delay. Active tapped delays, on the other hand, are much
more focused on digital applications, and so have less utility for
this necessarily hybrid system.
A preferred solution is to use programmable logic, in combination
with analog switches, to realize the purpose of a tapped delay line
24. These switches are implied (FIG. 1) in terms of the "enables"
associated with the buffers that drive the VCSEL elements. Instead
of replicating the signal 11 across many outputs of a tapped delay
line 24, however, a delayed version of a narrow control/gate pulse
is generated across many outputs of a programmable logic device (e.
g., field-programmable gate array FPGA) and used to gate the
enables on the VCSEL drive buffers 50.
Using this approach makes use of standard logic designs and also
enables the use of traditional semiconductor switch technology that
can be implemented as a gain enable, precluding the need for
relatively exotic analog tapped delay lines.
Because noise statistics are already set by front-end electronics
(cascade amplifier noise figures are dominated by the noise figure
of the first gain stage), the principal concern with the tapped
delay line 24 and associated switches 26 is the switch
rise-and-fall time. Good analog switches, such as the SN74LVC2G66
by TI, have nanosecond-class rise-and-fall times.
That TI unit, for instance, has switching times in the range of 1
to 5 ns, depending on drive voltage and rail voltage for the part.
This has the effect of limiting the temporal sampling and overall
bandwidth of the receiver as well. If for example the switch
temporal behavior is treated as a Gaussian with 5 ns width, the
equivalent bandwidth will be approximately 73 MHz.
In another preferred embodiment, the delay line 24 has clock
signals serving in lieu of a digital sweep signal. The delay line
24 responds to the clock signals by successively advancing the
successively received signals into and along the delay line
itself.
In other preferred embodiments dynamic RAM 88 or another capacitive
array receives the successive measurement signals distributed by
clock signals. Additionally, some preferred embodiments include use
of a shift register as the delay line wherein the memory elements
take on successive positions within the shift register and the
multiple signals 11 are read substantially simultaneously using
parallel circuits.
Vertical-Cavity Surface Emitting Laser (VCSEL)
In preferred embodiments, distribution of the digital signal can be
controlled using a VCSEL. In one example according to a preferred
embodiment of the invention, the VCSEL element of the receiver will
be assumed similar to a Honeywell SV3644-001 discrete VCSEL, as
this unit is a visible-wavelength output component that has been
successfully used.
As its characteristics are quite close to many published data
available for VCSELs, using this component as a representative of
VCSELs generally will not introduce significant error. Technical
specifications of interest for this Honeywell VCSEL are: 673 nm
output, 2 V threshold voltage, and 2 mA threshold current. It can
be driven above threshold, by as much as 4 to 10 mA, leading to a
0.12 to 0.30 mW output-power range.
The modulation bandwidth of VCSELs is generally very high,
extending beyond several gigahertz when the VCSEL is modulated
about some above-threshold quiescent point. Thus, the VCSEL is
band-limited only to the extent that its driver circuit is in that
mode.
The temporal response for bringing the VCSEL from below threshold
to above threshold is somewhat slow for multigigahertz telecom
applications (J. Tatum, J. Guenter, Modulating VCSELs, Honeywell
Application Sheet, 1998). At less than 1 ns, however, it is fast
enough for the systems being considered here.
Thus, even switching the VCSEL completely off via its bias 52
control will not introduce bandwidth limitations over those
introduced by the TIA 116. For instance, if a 2 ns overall
transient response is used (1 ns rise, 1 ns fall), then the
equivalent bandwidth for the VCSEL drive circuit will be 183
MHz.
Thus, while there is no immediate frequency limitation for the
VCSEL electronics, potential noise sources should be considered in
realizing a VCSEL-based receiver: 1. slope efficiency noise--The
slope efficiency of the VCSEL, which is a conversion gain
parameter, is approximately 0.6%/C; thus small changes in
temperature have a noticeable effect on gain. This is most likely
to be a dynamic-range issue more than a signal-to-noise-ratio
issue, owing to the likely high thermal mass of the diode relative
to the time scales of a single image/pulse event. 2. impedance
noise--As with slope efficiency, the change in impedance of the
diode will vary with temperature, approximately 0.3%/C. This is
also likely to be a slowly varying phenomenon, but will need to be
controlled or monitored, or both, to help with dynamic range. 3.
off-state bounce--For telecom applications, when a square pulse is
applied to the VCSEL, there can be a trailing edge "bounce" that is
some small fraction of the pulse height. This may be an impedance
mismatch issue, but should be kept in view during any system
development. The impact is likely to show up as amplitude noise on
the output-detector 12 element associated with a particular VCSEL.
4. relative intensity noise (RIN)--The RIN value for a VCSEL is
less than -125 dB/Hz. The RIN is caused by is coupling of
spontaneous emission from the laser into the stimulated emission,
introducing variation in optical power for particular combinations
drive voltages or currents. For the bandwidths in preferred
embodiments operating below 100 MHz, this is not a concern.
Hence neither bandwidth nor signal noise is an issue of concern for
the VCSEL, assuming adequate signal to modulate the VCSEL. The
VCSEL does, however, impact the dynamic range of the system and may
present a challenge to be addressed by customizing the system.
VCSELs can present a challenge for dynamic range because the
minimum and maximum output light levels above threshold are both
very bright, and not separated by even an order of magnitude--if
used in the way VCSELs are typically used for telecom applications.
Usually, the VCSEL is biased on at some quiescent current, and
modulated about this point.
For preferred embodiments of the present invention, to the
contrary, the VCSEL must be off when signal 11 is not present--so
that above-threshold operation occurs only when a backscattered
pulse 82 generates the appropriate signals. If this is not the
case, then fairly quickly (in the submicrosecond domain) the output
photodiode saturates: the full-well condition, for instance may
only be 100,000 e.sup.- for a CMOS detector 134.
Thus, one challenge in using VCSELs is to bias them just below
threshold, and bring them above threshold only when the desired
signal is expected. This is best facilitated using the same
programmable logic that provides the tapped delay-pulse signals for
gating the VCSEL switch/buffer 50.
By adding two digital control signals, the bias is readily
controlled. One signal, the bias enable, occurs just prior to the
VCSEL enable 52 pulse (the tapped delay signal) and releases just
prior to the falling edge of the VCSEL-enable 52.
The other signal controls integration of the final photodiode
detector 112, dumping charge from a given detector 12 (FIG. 5)
until just prior to the bias enable, and extending just beyond the
falling edge of the buffer enable. The VCSEL and photodetector 12
timing control thus cooperate to provide optimum dynamic range.
A conceptually direct way to combine the bias and signal inputs to
the VCSEL is to use a summing junction of a noninverting op-amp 16
(FIG. 6). If the digital signals are stable enough, it is possible
to add the VCSEL buffer-enable 50 to this driver--or to use the
bias control 53 itself as the enable, thereby simplifying the
overall circuit.
For a new implementation by the reader of this document, these
approaches should be tested and evaluated in the prototype phase.
This is not necessarily the most highly preferred circuit method in
terms of scalability, since a discrete transistor amplifier 16 may
be a better choice for each control point. The approach, however,
is conceptually correct and provides a means of control for a
single pixel prototype.
To mitigate temperature effects, either the temperature often must
be held constant or some measurements of VCSEL characteristics must
be captured between pulses 82 and then used to adjust system
parameters. For instance, it may be necessary to alter the VCSEL
bias 53 set-voltage as a function of time, to maintain closeness to
threshold conditions.
Image Photodiode
The image photodiode may be a CMOS detector 134 (FIG. 6), or can be
any of several available imaging sensor alternatives, including CCD
234 devices. A fairly high performance CMOS photodetector 134 to
use as an example is the IBIS4 sensor used in Seacam.
This component, according to its datasheet, has pixels with 50,000
e.sup.- full-well capacity, dark current of 787 e.sup.- and an
associated dark-current noise of 20 e.sup.-. Clearly, with such a
small full-well capacity, the VCSEL control mentioned above is
vital for achieving a useful dynamic range.
For instance, if the VCSEL is run for a typical streak-camera sweep
duration of 1 .mu.s, at the midrange VCSEL output of 0.21 W the
well attempts to accumulate 360 Me.sup.-. This represents an absurd
saturation condition.
As to noise sources: dynamic range is set by the dark current/noise
and the analog-to-digital converter (ADC) that is used. Most
likely, given the constraints on dynamic range that the VCSEL
ordinarily imposes, even an 8-bit converter preserves the available
postVCSEL dynamic range. Issues associated with noise, bandwidth
and dynamic range for each component in the signal chain are
readily analyzed (Table 1).
Representative values can be stated (Table 2) for a commercially
available point design of a front end according to preferred
embodiments of the invention, based on the following components:
Analog Modules 700 Series, Xilinx (e. g. model number XC9528), TI
(e. g. OPA686), Honeywell red VCSEL, model SV3644-001 (or for
visible operation the Avalon AP850), and a Fillfactory model IBIS4
CMOS array 134.
Experimental results (FIG. 3) have been acquired for point design
based on an airborne lidar application with APD. These represent a
terrestrial-target 64 case at 532 nm, using the Hamamatsu
Si-APD.
The indicated threshold level of 213 nW corresponds to a voltage
output of about 53 mV using the 2.510.sup.5 V/W sensitivity
specified in the Hamamatsu data sheet. The voltage corresponding to
the noise-equivalent power (NEP) is 9.7 mV and to the 90%
probability-of-detection (POD) signal level is 81.9 mV.
One limiting factor for preferred embodiments of the invention is
that of front-end analog bandwidth, and the noise spectral density
in this band. The streak camera 170 is able to achieve many
gigahertz of analog bandwidth with shot-noise-limited performance,
whereas current commercially available transimpedance amplifiers
116 suited for preferred embodiments of the present invention are
limited to a few tens of megahertz--for the highest sensitivity
applications.
This range can be extended through customization, as described
above and shown in the experimental results (FIG. 3). Thus limits
can be extended with customization through selection of optimal
components and settings.
Given the relatively high sampling rate provided by the back-end
electronics, however, it has been found that the front-end analog
bandwidth can be reduced without sacrificing system performance.
For example, the required bandwidth for a 10 ns pulse such as that
used for the demonstration STIL system 56, is 37 MHz. Using a
back-end sampling of 500 MHz (2 ns switching speed) this seemingly
low front-end bandwidth can yet yield a ranging performance of less
than a meter, akin to the subinch performance demonstrated with the
STIL using a 10 ns pulse 82--that would, superficially, seem to
preclude anything less than meter-scale performance.
Moreover, the invention still has a significant advantage over
streak-tube solutions where wavelengths beyond the visible (e. g.
700 nm or longer) or compact solutions (e. g. a few inches on a
side) are required. These are features that solid-state
technologies typically favor.
One preferred embodiment of the invention characterizes a
single-pixel sensor element for demonstration of three-dimensional
flash imaging. This embodiment can be scaled-up for multiple pixels
to form an N.times.N array.
A helicopter-borne flash-lidar system has been built and tested to
test an N.times.N array--specifically 64.times.64. Imagery 92
(FIGS. 7a through c) has been captured using this flash-lidar
system in field tests. This system uses multislit 144 streak-tube
imaging lidar (STIL) technology, which has been used for imaging
objects 64 as small as 1 mm and as large as 100 m in laboratory,
industrial, underwater and airborne settings.
As this is based upon streak-tube technology, however, as noted
earlier it has fundamental limitations in wavelength sensitivity
(visible-light photocathode 108 materials), physical size
(vacuum-tube structure) and relative fragility, and the need for
custom high-voltage bias and sweep electronics--as well as concerns
over the use of electron-tube devices in harsh environments. These
limitations preclude the use of MS-STIL flash-lidar systems in
applications that require, for instance, infrared (IR) sensitivity
and small size or weight, handheld configurations and the like--as
is the case for many missile-borne installations.
A preferred embodiment of the technology as part of a lidar system
(FIG. 9) uses a short pulse 82, near-infrared (NIR) eyesafe laser
83, projects a fan beam, and forms spatial-temporal 2D images of
the backscatter for each pulse 82 transmitted, with the third
dimension being added in pushbroom fashion. Here a single 2D image
is generated in a video display 92 of the terrain profile 80 imaged
with the lidar.
This lidar geometry (pushbroom, fan-beam) mimics many systems (FIG.
8) currently in use. Even more readily than in streak-tube-based
systems, the technology of the invention is extended to a
flash-lidar configuration.
In such a system a rectangular area is illuminated with a laser,
and the entire rectangular region range-resolved together, thereby
leading to a complete 3D image per laser pulse 82 and, if desired,
a range-resolved motion picture of the volume of interest. Using
the present approach, however, the flash-lidar capability is
inherent--it requires no special pixel-remapping optics such as
taught in the Alfano and Knight patents, and as is the case for the
MS-STIL flash lidar (this will be further explained in a later
section).
The next few paragraphs describe preferred embodiments of the
invention technology at an elemental level, and then extend the
concept to both the pushbroom lidar and flash-lidar cases. Also
included is some discussion of the way flash lidar is achieved with
conventional streak-tube technology, to highlight some advantages
of the present invention.
Additional detail on assembly of a receiver according to the
invention, using commercial off-the-shelf available components, can
be appreciated from a system conceptual design of a single
(spatial) pixel receiver (FIG. 10). A photodiode located at the
focal plane 172 of a lidar receiver intercepts photons 84 and
converts them to a continuing electronic signal (i.e., a time
series).
This signal is then amplified using a transimpedance amplifier 116,
so that it can be sampled using a tapped delay line 24--in which
each tap represents a temporal sample of the signal. This is
implemented, e. g., by sequential on-off switching of buffer
enables on a parallel set of signal amplifiers 16, one for each
VCSEL input.
An alternate approach is to incorporate the on-off switching into
the VCSEL bias circuit 52 (not shown, but standard in high-speed
applications of VCSEL technology). The effect of either choice of
on-off switching is the same: a temporal sample of the incoming
signal 11 is impressed upon the VCSEL, with the resultant generated
light modulation then representing a sample of the signal 11.
The signal samples drive a VCSEL array that, in turn, illuminates a
CMOS 134 image array. The image formed on the CMOS 134 image array
is the desired time-resolved lidar signal for a single spatial
pixel in the system focal plane. This signal can be readily
digitized and processed with relatively slow-speed electronics,
akin to what is commonly done in current mass-market digital
cameras.
This is extended to an additional dimension (FIGS. 9 and 13), in
the form of an implementation that can generate 2D spatial-temporal
images on a CMOS image detector 134. This implementation is
analogous to the STIL technology (covered in sections that follow)
that produces 3D images in a pushbroom fashion by fan-beam
illumination with repetitive laser pulses 82--using a streak camera
170 to form 2D spatial-temporal images for each pulse 82.
Thus, the present solution to 3D imaging provides the performance
needed from the streak tube using available semiconductor
components (this is shown more explicitly in the next section, see
Table 4), but without using any high-speed digitization. This is a
solid-state advance over the streak tube 170, which neatly combines
analog and digital electronics with optronics, leading to a very
small, high-performance 3D imaging solution.
Replacement of the pushbroom-style 3D lidar (FIG. 13) by a flash 3D
lidar configuration in preferred embodiments takes the form of a
photodiode array (FIG. 14) in the focal plane. The diode array is
followed by a TIA array that drives the delay line--and then a
VCSEL array that in turn illuminates the two-dimensional (2D) CMOS
array, where the 3D image is captured.
Here the line array of photodiodes (FIG. 13) gives way to a 2D
photodiode array (4.times.4 for tutorial and prototyping purposes).
This array, lying in the focal plane, drives a corresponding 2D
array of transimpedance amplifiers (2D TIA array) 116.
This TIA array in turn drives the input to the tapped delay lines
24, whose outputs are connected directly to VCSELs . The VCSEL
arrays provide light signals 11 to the 2D CMOS detector 134 array
where the full 3D flash-lidar image is captured (again, one per
laser pulse).
In practice these successive arrays are connected using
integrated-circuit and high-density interconnection technology that
is common within the electronics industry. Although this approach
is analogous to existing 3D imagers based on the streak tube--in
that it combines electronic and photonic technologies to circumvent
the need for high-speed digitization--this approach is different in
that it uses no electron tube, and requires no fabrication of
custom, high-speed, mixed-signal integrated circuits.
Preferred embodiments of these forms of the invention may be
hybrids of semiconductor photonics and electronics; they have the
small size of integrated-circuit technologies but the capability of
high-performance electron-tube systems, akin to streak-tube imaging
lidar. The next section explores this relationship further.
The Present Invention in Relation to Streak Tube 3D Imaging
The ensemble of technologies involved in the present invention can
be configured to behave as a 3D imaging system. The streak-tube
imaging lidar is an imaging streak camera configured to
time-resolve the backscatter of a line projected at range 68 (FIG.
15) by a pulse laser 83 source, leading--for each pulse 82--to a 2D
spatial-temporal image that can be captured with a 2D CCD 234 (or
like) array. The streak camera enables 3D imaging as follows.
1) The line image of backscattered light is formed on the
streak-tube photocathode 108, generating a corresponding line image
of photoelectrons within the tube. This electronic image is
accelerated toward the anode end of the streak tube, which is
phosphor coated.
2) The photoelectrons are electrostatically deflected (swept)
across the phosphor. The sweep action forms on the phosphor a 2D
image that has spatial (line-image axis) and temporal
(deflection/sweep axis) dimensions.
3) The CCD 234 camera captures the 2D image formed. Typically the
third dimension is generated in pushbroom fashion by repetitively
pulsing the laser--while moving the source-and-sensor platform
(FIGS. 9 and 15).
The tube geometry lends itself readily to a pushbroom lidar that
projects a fan beam (a line image at range), forms an image of this
fan beam on the photocathode 108, and then proceeds to
range-resolve the entire fan beam (line image) at once--leading to
the 2D temporal-spatial image for each laser pulse 82. If enough
room is available on the photocathode 108 for more than one line
image, however, then many lines--or equivalent fan beams--can be
imaged onto the photocathode 108 and range-resolved as an
ensemble.
This latter composite imaging is precursor to a flash lidar, which
in the case of streak-tube technology is in effect a group of
fan-beam lidar receivers all combined onto one streak tube. A
streak tube 170 can be used in this way to generate a flash-lidar
system (FIG. 16).
This is precisely the type of system that was used to generate some
imagery discussed earlier (FIG. 7). Here some elements are the
projection of a rectangular pulsed-laser light source and remapping
of the 2D focal plane onto streak-tube slits, or line images--and
in due course to full 3D images at the phosphor end of the streak
tube, for each shot of the laser.
In comparison to the STIL technology, the commercially available
off-the-shelf-components that comprise the signal chain, according
to preferred embodiments of the invention are: a. a high-speed
linear detector array 12 (e. g., InGaAs); b. a transimpedance
amplifier 116 array or hybrid package; c. a programmable tapped
delay element (e. g., surface acoustic-wave [SAW] device); d. a
vertical-cavity surface-emitting laser (VCSEL) array; and e. a 2D
image detector 34 (e. g., CMOS 134 or CCD 234 array).
Since the photodiode array is inherently discrete (as is opposed to
the continuous photocathode 108 of the streak 170 tube), the
present invention can be used in any of at least three distinct
operating modes: flying-spot (single-pixel) lidar, fan-beam
(pushbroom) lidar, or flash lidar. In certain most-highly preferred
embodiments of the invention, the flash-lidar configuration is
elected for the benefits of its instantaneous 3D lidar capability.
Nevertheless the present invention offers flexibility in choosing
approaches, which in itself can offer many advantages.
Although the present approach employs different technologies, it
follows an underlying sequence of events that is somewhat analogous
to that of the streak-camera systems (Table 3):
1) A line image of the backscattered light, formed on the
high-speed photodetector array 12 (e. g., InGaAs) is amplified and
distributed to many columns (one column per InGaAs pixel) of
switchable VCSEL drivers.
2) Each row of VCSEL drivers is enabled in time sequence, causing
the VCSEL array to emit photons in proportion to the signal present
from the InGaAs detection element.
3) The CCD 234 or CMOS 134 array captures the 2D image formed.
A performance comparison for STIL and now-preferred receiver
technologies may be noted (Table 4). The weight and volume
estimates are based on a 64.times.64 spatial-element flash lidar,
with decimeter range 68 resolution. The STIL technology estimates
assume a military camera based on a Photonis P930 streak tube and
custom electronics.
The invention estimates are based on commercially available
off-the-shelf components and engineering figures for required
packaging/integration. First-order calculations for sensitivity,
bandwidth and packaging indicate that the invention matches the
streak-tube performance in many of the relevant parameters. It is
known, however, that the photodiode-TIA combination can be
optimized for noise performance by using a customized TIA 116.
A person skilled in the art can appreciate that because the system
utilizes mainly commercially available technologies as its core
elements, and because prototyping and testing of this technology
can be accomplished using existing RF and high-speed circuit design
techniques (without resorting to custom silicon fabrication), the
present system is easily adaptable to various applications and
customized performance levels.
Preferred embodiments of the invention nevertheless can be based on
customized parts. In one such assemblage, a readout module 190
(FIG. 17a, courtesy of Irvine Sensor Corporation) has an
infrared-detector line array (64.times.1 channel wideband
interface). A printed-circuit board 192 carries a CMOS 134 or CCD
234 imager (64.times.64).
The imaging board 192 and readout module 190 are advantageously
positioned very close together and sealed in place. Preferably the
PC board 192 is very large (not shown), and includes electronics
and sockets for a readout-module tape cable 194--as well as
physical support for the readout module 190.
The tape cable 194 (FIG. 17b) is advantageously attached to the
board 192. In preferred embodiments the tape-cable module interface
194 is ball-grid-array-bonded to the module and connector.
In preferred embodiments this interface is a 64.times.64 VCSEL
array, bump-bonded to the module. The VCSEL output falls on a
matched imaging-array combination, which captures the amplitudes of
all the incoming pulse segments.
The module 190 components (FIG. 17c) include two passive silicon
caps 198 in the module--as well as active sixty-four silicon slices
200 of 30 to 100 microns each, to match the pitch of the VCSELs.
Bus stripes 202 interconnect clocks and power control to all
layers.
The module is connected to a line array of detectors on 30- to
40-micron centers, wirebonded to the module 190. Metalization on
the module face can be used to compensate for differences in
detector 12 and VCSEL spacing.
Each single slice of the readout module 190 (FIG. 17d) has mosaic
VCSEL bumps 196 bound on top of the module 190, and input/output
("I/O") connections 204 that pass through the side of the layer for
a bus interconnection. In this exemplary embodiment the detector
line array 12 connects at the bottom of the slice. Electronics
include a detector buffer, common electronics, timing, and
sixty-four channel output VCSEL drive electronics.
Single-pixel Embodiment Evaluation
In customizing parts for optimization or testing of preferred
embodiments it is advisable to begin with the design of a
single-pixel embodiment to address design issues down to the
component level through analysis, simulation and limited
prototyping. To build a robust elemental (single pixel) system, the
following features should be tested for optimization. Workers in
this field will understand, and will be able to and carry out, such
testing: 1. assessment of commercially available component
technology for application to development of the invention in terms
of availability for prototype, and applicability to terrestrial and
marine lidar applications-- a. photodiode arrays: impedance
matching, noise performance and dynamic range; b. transimpedance
amplifier 16 arrays: evaluation of bandwidth, packaging, impedance
matching, and low-noise design limitations; c. VCSEL arrays:
evaluate bandwidth, threshold, dynamic range, and power
consumption; d. delay line 24: technology options, noise behavior,
and bandwidth; 2. characterization of analog performance for the
photodetection-transimpedance-emitter signal chain-- a. gain, phase
and transient response for each element (photodetector 112,
transimpedance amplifier 116, and VCSEL 30); b. front-end
integrated noise performance; c. gain, phase and transient response
for the ensemble; 3. assessment of the scalability of the elemental
design to larger N.times.N arrays, leading to both pushbroom and
flash-lidar systems-- a. evaluation of the potential for an
applicable N.times.N prototype; and b. system design of an
N.times.N (e. g. N=4) prototype for implementation.
The single-pixel sensor embodiment or its optimization should be
tested using a model or single-pixel subassembly. This is best done
by building a model, using design software for a standard
analog-signal and mixed-signal-circuit simulation--as for example
building a PSPICE.RTM. model of the signal chain--to provide a
basis for evaluating and understanding the experimental
results.
Next it is advisable to build a printed-circuit board (PCB) test
fixture that allows for placement and testing of individual
components, and the ensemble of components that make up the entire
single-pixel channel 58. Test-fixture connectors should accommodate
all the test instruments to be used (e. g., SMA for network
analyzer, fiber connect for streak camera and test laser), and
should include subcircuits for:
a. the front-end photodetector 112,
b. the transimpedance amplifier 116, and
c. the VCSEL photoemitter driver.
The fixture should also enable these components to be connected
together and characterized as a system. The characterization can be
made for the subcircuits individually and for the system as a
whole, and preferably includes characterization of:
a. d. c. current and voltage behavior,
b. temperature dependence of gain and phase,
c. signal gain and phase, and
d. noise and dynamic range.
Multipixel Embodiment Evaluation
The preferred multipixel embodiment can be optimized or tested
using the measured and modeled results of the single-pixel
embodiment evaluation and existing commercial technology (FIG. 11)
to expand the system design to encompass an N.times.N array design
(e. g. FIGS. 13 and 17). In this preferred method for testing or
optimization of the system, evaluation of a pair of the
single-pixel embodiments is recommended so that channel-to-channel
interactions are taken into account in designing the N.times.N
array.
Also recommended is the use of a pair of linear stage positioners
to capture 3D imagery. These indicate the potential for scaling of
the single-pixel design. It is also preferred that the N.times.N
evaluation examine system power, sensor noise, channel
cross-coupling and other component-level interaction. Thus a
reasonably complete characterization can be made/deduced from the
single-pixel embodiment as applied to an N.times.N array, as will
appreciated by workers skilled in this field.
As suggested earlier, one objective of this invention is
manufacturing economy. Accordingly it is advisable to seek
implementation of the invention in configurations that can be
manufactured as inexpensively as possible without significantly
impairing performance.
In certain of the accompanying apparatus claims generally the term
"such" is used (instead of "said" or "the") in the bodies of the
claims, when reciting elements of the claimed invention, for
referring back to features which are introduced in preamble as part
of the context or environment of the claimed invention. The purpose
of this convention is to aid in more distinctly and emphatically
pointing out which features are elements of the claimed invention,
and which are parts of its context--and thereby to more
particularly claim the invention.
It will be understood that the foregoing disclosure is intended to
be merely exemplary, and not to limit the scope of the
invention--which is to be determined by reference to the appended
claims.
TABLE-US-00001 TABLE 1 Issues associated with noise, bandwidth and
dynamic range for each component in the signal chain. Element/Issue
Noise Bandwidth Dynamic Range Photodiode Dominated by shot noise
Limited by equivalent Limited by the detector (quantum efficiency)
diode capacitance and the size and applied bias closeness of the
amplifier to an ideal current source TIA Set by amp intrinsic noise
Limited by gain bandwidth Limited by NEP, voltage/current, source
product, feedback resistor, output rail voltage impedance,
bandwidth diode capacitance Distribution Amp Set by noise figure,
but as part Limited by gain bandwidth Limited by IP3, output of
system noise will be mitigated product rail voltage by TIA gain.
Buffer/Switch Small charge injection noise may Switch enable
rise/fall times Set by rail voltage and be an issue switch
conductance in on state VCSEL Shot to shot amplitude noise is
Limited by the threshold transition Limited by ability to likely to
dominate; second oreder (rise/fall) times for on-off control bias
near edge effects will include thermal and modulation; limited by
device of threshold and laseremission (amplitude) noise. intrinsics
for CW modulation linearity and (many GHz). repeatability of on-off
. modulation of the laser Photodiode Readout, dark noise will
dominate, Not an issue, as this element will Limited by VCSEL given
significant light output be an integrator, or low pass filter.
dynamic range on low of VCSELs. end, well depth on high end.
TABLE-US-00002 TABLE 2 Representative values for a commercial
over-the-counter technology front end according to a Element/
Dynamic Issue Noise Bandwidth Range Photodiode >30% QE >1 GHz
>20 dB TIA >8 fW/ Hz <200 MHz 100,000 (2V/2e-5V)
Distribution / Hz Est same at TI Limited by Amp opamp IP3 Buffer/
Small charge 73 Mhz Set by rail Switch injection noise voltage may
be an issue VCSEL TBD noise from 182 MHz 6 mA/ thermal, 0.1 mA = 60
threshold/ wavelength Photodiode >1 MHz ADC/ >6 bits readout
(VCSEL limited, >8 bits otherwise)
TABLE-US-00003 TABLE 3 Analogy between preferred embodiments of the
present invention and streak camera systems for 3D image
generation. Streak Camera Function Component HESS Component
Photon-photoelectron Photocathode GaAs detector array conversion
(e.g. S10) Photoelectron gain Multi-KV cathod/anode Transimpedance
tube biss amplifier array Controllable time Electrostatic sweep
CMO5 or SAW resolution electrodes delay line Photoelectron-photon
Phosphor (e.g. P20) VCSEL array conversion Image capture CCD camera
CCD, CMOS, or other array
TABLE-US-00004 TABLE 4 Comparison of receiver technologies for
performance between STIL and preferred embodiments of the present
invention. Parameter STIL HESS (COTS) Minimum focal plane pixel
size 20 .mu.m 50 .mu.m Maximum radiant gain 15 dB >30 dB Visible
quantum efficiency (530 nm) 10% >30% IR quantum efficiency (1500
nm) 0% >80% Maximum temporal resolution 2 ps 20 ps Packaged
receiver volume 5000 cm.sup.3 1000 cm.sup.3 Packaged receiver
weight 4000 g 600 g
* * * * *
References